Electrical impedance tomography (“EIT”) is an imaging modality that evaluates the conductivity within the interior of objects using resistance measurements obtained from electrodes/sensors on that object's outer surface. In the typical measurement, currents are injected into the object through electrodes placed on the surface of the object, with voltages measured at other electrodes. Considerable effort has been invested in determining current distributions that will provide best clarity in viewing and conductivity profile within the object. Input currents are defined at each electrode, voltages are measured at each electrode, and then the conductivity profile of the object is reconstructed. This reconstruction process is an inverse problem that is considerably more difficult than many other imaging modalities such as x-ray computed tomography because the current paths in an EIT measurement are able to greatly adjust themselves to an object's interior conductive profile. Another problem with conventional EIT measurement is that large changes at localized sections in the object's interior will produce only small changes in measured voltages at the exterior. In this regard, a challenging task for EIT is to reconstruct the internal impedance profile given low level, noisy voltage measurements. Conventional aspects of EIT are discussed in “Electrical Impedance Tomography,” IEEE Signal Processing Magazine, November 2001, which is incorporated by reference herein.
Systems such as adaptive current tomography (“ACT”) have been developed to provide an impedance image for a body. In ACT, electrodes are placed on the exterior of a body and currents are then applied simultaneously to each electrode. Electrode voltages are then measured to generate the data required to perform an image reconstruction. These electrodes may be placed in a single plane or in several layers. A third generation ACT system (which is denoted as ACT3) uses 32 electrodes, and the applied currents are 28.8 kHz sinusoids, resulting in a system that can measure both lossy and reactive components of the impedance. This ACT instrument is real-time and is capable of producing about 20 images per second. The ACT3 system has some shortcomings. For example, it is difficult to: (1) accurately discern impedance anomalies in the interior of an object; (2) accurately evaluate the impedance values of these anomalies; and (3) spatially resolve the anomaly in an accurate manner.
Numerous algorithms have been proposed to use electrical measurements derived from EIT to reconstruct an object's conductivity profile. A typical approach for determining the conductivity profile within an object is to model the object as a resistive network. The discrete resistors within this network are treated as unknowns. The resistance value of each of the discrete resistors is determined by minimizing the least squares error in the voltage measurements at each electrode.
The measure or degree with which a measurement system can detect an impedance anomaly is referred to as distinguishability. Measurement systems with greater distinguishability should generally provide improved image data for an observed body. Generally an algorithm or measurement process that has more distinguishability will provide more superior conductive profile reconstruction. Thus, high distinguishability is a desirable trait in EIT systems.
Accordingly, it is desirable to have an EIT system that addresses the above shortcomings of conventional measurement techniques. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.